1. Field of the Invention
The present invention relates to a pneumatic tire provided with a tread pattern having a land portion in which at least one sipe is formed, and is particularly useful as a studless tire.
2. Description of the Related Art
Conventionally, in a studless tire, a cut called a sipe is provided in a land portion of a block, a rib or the like, and a stable travel can be achieved on an ice road surface having a low wear coefficient on the basis of an edge effect generated by the sipe. As the sipe mentioned above, there has been known a so-called two-dimensional sipe in which a shape is not changed in a depth direction, and there is put for practical use a flat sipe in which a sipe inner wall surface is flat, and a waveform sipe 20 in which concave and convex rows extend in a normal direction ND of a tread surface as shown in FIG. 3.
In recent years, the number of the sipes tends to be increased for increasing an edge effect, however, if the number of the sipes is increased too much, a rigidity of the land portion is lowered and the land portion collapses excessively in spite that the number of the edges is increased. Then there is generated a problem that the edge effect becomes small, whereby an ice performance is lowered, or a stepped wear is generated between the sipes. Accordingly, a so-called three-dimensional sipe in which an excessive collapse is suppressed by changing a shape of the sipe in a depth direction has attracted attention.
There has been proposed various shapes as the shape of the three-dimensional sipe, for example, a sipe extending in a wavy shape on a sipe bottom while extending in a linear shape on a tread surface is described in Japanese Unexamined Patent Publication No. H9-183303. Further, in Japanese Unexamined Patent Publication No. 2006-27558, there is described a sipe in which the concave and convex rows of the sipe inner wall surface are inclined in a zigzag shape in a longitudinal direction side of the sipe with respect to the normal direction ND of the tread surface, such as a sipe 30 shown in FIG. 4. Further, in Japanese Unexamined Patent Publication No. 2004-161166, there is described a sipe in which a sipe inner wall surface is divided into two or more sections in a depth direction, and the number of the waves of the sipe in each of the portions is increased step by step toward the sipe bottom side.
However, in the sipe described in Japanese Unexamined Patent Publication No. H9-183303, an engaging effect of the concave and convex rows is small, and it is impossible to sufficiently suppress the collapse of the land portion. Further, in accordance with the sipe described in Japanese Unexamined Patent Publication No. 2006-27558, the convex stripe and the concave stripe extending in a diagonal direction are engaged at a time when the land portion collapses, whereby the excessive collapse is suppressed, however, since there is a tendency that a comparatively soft rubber is used for the tread rubber in recent years, it is necessary to execute a further improvement for sufficiently obtaining the effect of suppressing the collapse of the land portion.
Further, in the sipe described in Japanese Unexamined Patent Publication No. 2004-161166, since an end portion of the convex stripe or the concave stripe is arranged in a boundary where the number of the waves of the sipe is changed, a lower end of the convex stripe is engaged with an upper end of the convex stripe provided in the sipe inner wall surface opposing thereto in the depth direction, and the engaging position is provided along a longitudinal direction of the sipe, whereby there is a case that the edge effect generated by the sipe is lowered. In addition, there is a problem that it becomes very hard to manufacture a blade for forming the sipe and release a mold due to the concave-convex shape of the sipe inner wall surface mentioned above.